The present invention relates to forming a compound material using a plastic-system compound material, and especially using a prepreg material and the like.
A pull-forming method is one example of a method for continuously manufacturing a molded material or a cylindrical material from a composite material. Such forming method generally involves impregnating resin to reinforcing fibers such as carbon fibers or glass fibers, and sending the material through a heatedmold, so as to cure the resin and to continuously produce molded material. Such method is low in cost, but has problems in the strength, the stability, the reliability of the secondary adhesion and the like of the product.
A method solving the above-mentioned drawbacks of the pull-forming method is proposed in Japanese Patent Nos. 1886522 and 1886560 owned by the present applicant.
The materials disclosed in the above patents have the characteristics of adhering completely to other structural materials. Therefore, they may be used advantageously as the structural material of an aircraft.
However, the materials disclosed in the patents had to be improved of their compressive strength, their load/displacement characteristics and the like in order to be used as the compound material utilized as the structural material of an aircraft.
In order to solve these problems, there is a need to improve the deterioration of the compressive strength of the material which is caused by very fine wavy deformation of the fibers placed inside the material as reinforcement members.
In many cases, the compound material utilized as the structural material of an aircraft includes carbon fibers aligned in one direction (unidirectional fiber) as the reinforcement member to improve the strength of the material.
Analysis of the microphotograph of the compound material has proved that the unidirectional carbon fibers are repeatedly displaced perpendicularly to the direction of arrangement of the fibers (wavy deformation).
The details of analysis of the compound material 10 will now be explained with reference to FIG. 9.
The direction of the compound material 10 is shown by arrow xe2x80x9caxe2x80x9d. The unidirectional fibers (carbon fibers, glass fibers and the like) 15 positioned inside a plastic material 11 should be arranged parallel to arrow xe2x80x9caxe2x80x9d, but as shown in the drawing, the fibers 15 are bent in a wavy manner against the arrow xe2x80x9caxe2x80x9d direction.
A state is shown in FIG. 10 where the compound material 10 including the wavy-deformed fiber 15 receives compressive strength. When the deformed, decentered fiber 15 receives compressive load P from the direction of the fiber, the fiber will oppose to the load up to a predetermined level, but when the load exceeds the limit of the fiber, the fiber will be buckled (refer to FIG. 10(a)).
As shown by FIG. 10(b), when further load P is placed, the buckled fiber 15 will be decentered to eccentricity xe2x80x9caxe2x80x9d by even a small load.
As explained, the deformed fiber 15 was easily decentered by a small compressive load, and did not exert the expected compressive strength, which was disadvantageous for a structural material.
Moreover, when a decentered fiber exists in the completed structural member, the decentered fiber existing in the resin will be straightened at first when tensile load is provided to the material. After the fiber is straightened, the compound material will show reaction force. The state is shown by a stress/strain diagram (refer to FIG. 11). The relation between the load (compressive stress, tensile stress) and the strain (shrinkage, elongation) is not expressed by a straight line at the initial point 0. In other words, the structural member will not be elongated in proportion to the tensile stress.
As a result, the stress is concentrated to the fibers having a relatively high straightness, and the fiber to which the stress is concentrated may break, which results in the deflection of the material.
The cause of existence of such decentered fiber is that in the prior art forming method where pressurization and depressurization is repeated intermittently, pressure could not be added continuously during the curing of the resin, and the fine waves of the fibers included in the material could not be straightened during the forming process.
The completed material which has gone through the forming process is cooled, and returned to ordinary temperature. At this time, the thermally expanded resin will shrink.
During cooling time, when thermal shrinkage occurs to the structural material with the reinforcement fibers being elongated linearly, the fibers oppose the thermal shrinkage of the resin and maintain a linear state.
For example, the thermal expansion coefficient of an epoxy resin used as the matrix of a structural member is as large as 27 ppm/xc2x0 C. When the temperature of the product is cooled down from 180xc2x0 C. to 20xc2x0 C., the structural material will shrink by 0.4%. On the other hand, the thermal expansion coefficient of the included carbon fibers is as small as 0.3 ppm/xc2x0 C. (5 ppm/xc2x0 C. in the case of glass fibers), and the fibers hardly shrink. In other words, during thermal shrinkage, the fibers which are not elongated linearly will be bent even further accompanying the shrinkage of the resin. As a result, as shown in FIG. 9, the fibers acting as the reinforcing member of the structural material are deformed to have continuous wavy bends.
According to calculation, a thermal shrinkage by 0.4% will result in the deformation of fiber having a wavelength of approximately 3 mm and an eccentricity rate of 1.6%, which cannot be ignored.
The drop in compressive strength of the material caused by the fine wavy deformation of the fiber included in the material as reinforcement member is a problem common to any compound structural material of the prior art. In the studies, such a problem is called micro-buckling, and the method for solving such a problem is required.
The present invention solves the problem by providing a method and device for forming a compound material characterized in that the fibers included in the compound material as reinforcement member are not deformed to have waviness, but instead, arranged linearly within the material.
In order to solve the above-mentioned problem, the present invention provides a method for forming a compound material comprising a pinching step of pinching and fixing the upstream end and the downstream end of a composite material and providing a tension toward the downstream advancing direction to the downstream end of said composite material, a heating/pressurizing step of heating and pressurizing the pinched and fixed composite material intermittently, and a moving step of releasing the compound material and sending said material to the downstream direction after termination of the heating/pressurizing step and setting the amount of movement of said compound material at the downstream side to be greater than the amount of movement of said compound material at the upstream side.
In a further aspect of the method, the tension toward the downstream advancing direction provided to the downstream end of the material is added from before the starting of said heating/pressurizing step to after the termination of said heating/pressurizing step. In another aspect of the method, during said moving step, a tension toward the upstream direction is provided to the upstream side of said compound material.
The device for forming the compound material according to the present invention comprises a pinching means for pinching and fixing the upstream end and the downstream end of a composite material being supplied thereto and further equipped with a tension adding means providing tension toward the downstream direction of movement, and a heating/pressurizing means for heating and pressurizing said composite material being pinched and fixed, wherein a downstream moving means is set so that the amount of said compound material being moved by said downstream moving means is greater than the amount being moved by an upstream moving means, and the tension adding means of the pinching means is started just before said heating/pressurizing means is started.
Even further, the upstream moving means is equipped with a tension adding means for providing tension toward the upstream direction opposite to the advancing direction, the tension toward the downstream direction of said downstream moving means being set to be greater than the tension provided by said tension adding means of the upstream moving means, so that the amount of movement of the composite material by the downstream moving means is greater than the amount of movement of the material by the upstream moving means. In another aspect of the invention, the tension adding means of the pinching means pinching the downstream end of the material providing tension toward the downstream direction starts to operate when said upstream moving means and said downstream moving means stop operating, and said tension adding means stops operating when said pinching means pinching the upstream end and said pinching means pinching the downstream end stop operating. During all molding steps, the fibers in the material are provided with tension so that they are straightened linearly in the material.